Inkjet printers have the advantages of low noise and low running costs and thus are becoming widely available recently. Also, color inkjet printers capable of printing on regular paper are available in the market. However, it is still difficult to meet all of the requirements associated with color printing, including color reproducibility, water resistance, light resistance, dry fastness, feathering, bleeding, and ink discharge reliability. Two-color composite portions of two of red, green and blue colors are more likely to undergo quality degradation particularly where a color printer is used, although monochrome portions of yellow, magenta or cyan color do not undergo quality degradation in this case. In particular, when a printed image is to be dried without using any fixing device, the ink dry speed is increased by enhancing the ink permeability of paper (Patent Literature 1). With this approach, however, outstanding bleeding occurs in the printed image.
Patent Literature 2 discloses that ink containing dialkylsulfosuccinate as a surfactant increases the ink dry speed and thereby reduces image quality degradation. Unfortunately, the ink offers different dot diameters for different types of paper, significant image density reduction occurs, and when it is exposed to alkaline conditions, decomposition of the ink's activating agent occurs that results in the loss of the agent's effect during storage.
Patent Literature 3 discloses ink that contains strong basic substance. This ink is effective for rosin-sized acid paper, but not for paper manufactured using an alkylketenedimer or dialkylsulfosuccinate as a size agent. Moreover, even printed on acidic paper, this ink is not effective in two-color composite portions.
Patent Literature 4 discloses a recording liquid containing a polyalcohol derivative and pectin. Pectin is added as a thickener for the purpose of preventing bleeding. However, pectin is a non-ionic compound having hydroxyl groups as hydrophiilc groups and thus undesirably reduces the discharge stability after downtime of the printer. To overcome this problem, at present, multi-pass printing is employed upon color image printing so that the amount of ink that permeates paper is suppressed so as to attain increased image density. To achieve higher-speed printing, there is an attempt to suppress the level of ink that permeates paper along its thickness at positions where secondary colors are to be printed.
Patent Literatures 5-7 each disclose an improved ink set, wherein black pigment ink is employed as black ink to suppress its permeability to paper as well as to increase image density, and dye inks are employed as color inks with some degree of permeability to paper, whereby in a printed image color bleeding is avoided at boundaries between black and other colors. Neither of the inks can fully prevent bleeding when printed at high-speed; therefore, they are not satisfactory.
Patent Literature 8 discloses an ink set wherein an ink containing a pigment-based colorant undergoes aggregation or thickening by contact with an ink containing a dye-based colorant to prevent bleeding at color boundaries of image. Portions of image recorded using an dye-based ink are inferior in water resistance and light resistance to those recorded using a pigment-based ink, impairing the color balance over the entire image with time.
In Patent Literature 9, the inventors proposed an ink set that can provide a sharp color image by high-speed printing while preventing bleeding at color boundaries. However, there still remains a need to improve the ink set so that it can prevent bleeding and beeding when applied on regular paper with remarkably poor ink absorbability or on a coating medium with no aqueous ink-receiving layer.
To ensure ink discharge reliability of the recording head, Patent Literature 10 discloses employing ink surface tension γL which is smaller than the critical surface tension γL at the nozzle surface; Patent Literature 11 discloses using ink with a surface tension larger than the critical surface tension of a member surface; and Patent Literature 12 discloses a technology wherein the inner diameter of the ink discharge tube, ink surface tension, ink density, and waste ink thickness are specified.
With these proposals, it is still difficult to ensure sufficient discharge reliability for inks that have high pigment concentration or high solid content (high resin content) when mixed. An ink set system where inks thicken or aggregate by reaction upon mixed together requires a plurality of flow members in order to prevent unwanted blending of waste ink, thereby requiring a complicated mechanism.
Patent Literature 13 discloses a technology wherein both of the ink surface tension and adhesion tension between the ink and recording medium are specified for the purpose of improving the ink fixing property with respect to the recording medium.
The disclosed ink with improved fixing property is capable of ensuring storage stability when stored in a hermetically-sealed container. Nevertheless, the ink is not so stable in a system where ink vehicles are evaporated, such as in an open waste-flow path. Thus, there still remains a need to improve its discharge reliability.    [Patent Literature 1] Japanese Patent Application Laid-Open (JP-A) No. 55-29546    [Patent Literature 2] Japanese Patent Application Publication (JP-B) No. 60-23793    [Patent Literature 3] JP-A No. 56-57862    [Patent Literature 4] JP-A No. 01-203483    [Patent Literature 5] JP-A No. 2001-55533    [Patent Literature 6] JP-A No. 2004-339489    [Patent Literature 7] JP-A No. 2004-352996    [Patent Literature 8] JP-A No. 2004-197055    [Patent Literature 9] JP-A No. 2003-113337    [Patent Literature 10] JP-A No. 04-372669    [Patent Literature 11] JP-A No. 2002-273888    [Patent Literature 12] JP-A No. 2001-1553    [Patent Literature 13] Japanese Patent (JP-B) No. 3733149